In order to satisfy favorable economics for the refining of crude oil, it is often necessary to blend two or more crude oils prior to carrying out the various refining processes. However, there are particular problems associated with blending crude oils. One major problem is that crude oils are often incompatible with each other resulting in equipment fouling and ultimately equipment shutdown. Such equipment includes, but is not limited to, pipes, tanks, heat exchangers, furnaces, fractionators and reactors. Another major problem with blending crude oils and other hydrocarbons is the production of oil and water emulsions in the crude oil slop system preventing the oil slop from being processed by refinery equipment, such as crude distillation units. Another major problem is the production of emulsions in crude desalter units often having a deleterious effect upon the waste water system associated with the desalter unit. In light of these problems, crude oil incompatibility has been plaguing the refining industry for many years resulting in lost profits due to unnecessary equipment shutdown and limitations on the crude oil slate available for refining.
The primary culprit that causes incompatibility of crude oils is the presence of organic solids in the form of precipitated asphaltenes in blended crude oils. Current theory regarding the asphaltene-crude oil relationship postulates that such relationship is similar to a solute-solvent interaction wherein a certain solvent strength is required to hold asphaltenes in solution in crude oil. The primary parameter governing the ability of asphaltenes to remain in solution in crude oil is the aromatics to saturates ratio of the crude oil. It is known that asphaltenes are soluble in aromatics such as toluene, but insoluble in paraffinic compounds such as n-heptane. Accordingly, asphaltenes are defined herein as the non-volatile and polar fraction of crude oil that is insoluble in n-alkanes.
The underlying problem associated with the presence of asphaltenes in crude oils is that asphaltenes frequently precipitate from solution during the blending of two or more incompatible crude oils. This is generally thought to be caused by perturbations of the indigenous crude oil composition disrupting the delicate balance that keeps the asphaltenes soluble in crude oil. It is also believed that oil-water emulsions are formed and stabilized in part by the presence of precipitated asphaltenes from incompatible crude blends. Consequently, when left unchecked, asphaltene precipitation manifests itself in a variety of undesirable areas, including refinery equipment through the formation of coke and the generation of oil-water emulsions in storage tanks.
In the past, crude oil compatibility could be determined through extensive laboratory testing. For blends of two crude oils, the determination of crude oil compatibility is relatively straightforward since the number of tests required to define the acceptable blend ratios is relatively small. However, for each additional different stock of crude oil added to a blend, the number of lab tests required to ascertain the range of incompatibility goes up exponentially making the determination of crude oil compatibility intractable. This presents a difficulty when economic conditions justify blending three or more crude oils together for feed to crude distillation units or other refining processes. Accordingly, there is a need for a practical and cost efficient means for determining the viability of blending different crude oils.
In response to this need, the petroleum refining industry has devoted extensive resources and effort to develop new methods to solve the problem of blending different crude oils. However, such efforts have only partially succeeded in providing a practical yet cost effective method for blending different crude oils.
One such effort is U.S. Pat. No. 4,843,337 issued to Dickakian et al., which discloses a method for blending hydrocarbon liquids at a ratio to maintain the combined aromatic to asphaltene ratio above a certain predetermined level to prevent fouling of process equipment. However, the Dickakian disclosure is limited to a method for blending two hydrocarbon liquids leaving unsettled the problem of blending three or more crude oils.
U.S. Pat. No. 5,871,634 and U.S. Pat. No. 5,997,723, both issued to Wiehe et al., disclose a method for blending potentially incompatible crude oils by combining each crude oil in order of solubility blending numbers such that the solubility blending number of the mixture is greater than the insolubility number of any crude oil of the mixture. However, the Wiehe disclosures teaches a method that employs inexact and onerous laboratory tests, such as conventional optical microscopy or crude oil filtration to determine the presence of asphaltenes in each crude oil. Moreover, the Wiehe disclosure employs a complex blending and titration analysis to determine the insolubility number and the solubility blending number for each crude oil.
Although the foregoing disclosures provide advances in the art, there is still a need for a method for accurately determining crude oil incompatibility that is practical and cost efficient.
It has also been found that centrifuging one or more crude oils blended with predetermined amounts of heptane provides for a simple yet cost effective means for determining asphaltene instability and the amount of asphaltenes in each crude oil.
It has also been found that the relative ratio of aromatics to saturates in each crude oil to be blended can be easily determined by using the relationship between the boiling point and the density of each crude oil.
It has also been found that compatible blends of two or more crude oils can be determined based on the relationship between the boiling point and the density of each crude oil in the blend and the determination of asphaltene instability in each crude oil in the blend.